Sound, e.g. music, normally consists of a wide range of frequencies which contain also bass components extending over a bass frequency range. The bass components play an important role in music. Thus, for example, a piano keyboard covers a wide range of frequencies with more than a quarter of its notes allocated to frequencies below 200 Hz. Various "sound effects" that are used in film, video clips, multimedia ("MM") games and others, contain frequencies below 100 Hz; typical examples being the sound of cars, helicopters, motor cycle engines, cannon fire, bomb explosions etc. (see also Ref. 2. FIG. 2-4 pp.18-19).
Whilst, as specified, bass plays an important role in sounds, there are inherent difficulties in reproducing acoustically (i.e. through elecrto-acoustic transducers) low frequencies and accordingly, the resulting low-frequencies as sensed by the listener (i.e. low frequency psyco-acoustic sensation) is adversely affected owing to physical limitations of the electro-acoustic transducer and psychoacoustic reasons.
Consider, for example, the sound of a piano. This sound contains low and high frequencies which give rise to low and high frequency psyco-acoustic sensation respectively.
Reproducing a recording of such a sound through electro-acoustic transducers will degrade the low frequency psychoacoustic sensation due to physical and psyco-acoustic limitations associated with the low frequency.
As to the physical and psyco-acoustic limitations, the former resides in the inherent low efficiency of electro-acoustic transducers in the low frequency range. The specified low efficiency stems from the relatively small size of the transducer, relative to the acoustic wavelength of the low frequencies. Thus, the wavelength of low frequency acoustic waves in the range of 20-300 Hz is between 10 to 1 meters, respectively. The physical size of the electro-acoustic transducer is, normally, much smaller than the wavelength in the low frequency range and in some cases the size of the electro-acoustic transducers amounts to about 1/100 of the low frequency wavelength. This physical shortcoming results in that the efficiency of the electro-acoustic transducer in the low frequency range is significantly lower as compared with its efficiency in the higher frequency range, thereby degrading the low frequency psyco-acoustic sensation (LFPS).
The other aspect which further degrades the LFPS resides in the field of psychoacoustics. As is well known in the psychoacoustic field, frequencies below about 300 Hz (and above about 5,000 Hz) are treated by the ear's physiology in a non-linear manner. Consequently, in a complex tone sound, (i.e. containing a wide range of frequencies), the balance between low, medium and high frequency ranges changes as a function of the overall sound level. Thus, when lowering the overall sound pressure level (SPL) of music the loudness level of the mid-high frequency range will be attenuated correspondingly whereas the loudness of the low frequency range will be attenuated by a different, higher factor. Therefore, it is difficult to control the manner in which the listener will perceive the loudness attribute of the low frequency psychoacoustic sensation (LFPS). (see also Ref. 1. ch 2.3 pp. 46)
Prior art techniques attempt to cope with degradation of LFPS, i.e. by compensating for the physical and psyco-acoustic limitations by means of treating the low frequency range signal.
The degradation in the LFPS due to physical and psychoacoustic factors as discussed herein is exhibited in commercially available electro-acoustic transducers, and particularly in the so-called desk-top multimedia speakers.
Desk-top MultiMedia speakers are normally coupled to a conventional personal computer (P.C.) and are characterized by being small in size, (due to the limited physical space that is allocated therefor on the desk-top). Desk-top-MM speakers are normally designed to work in a relatively low overall loudness level and are generally of a medium or low quality, as compared to conventional home stereo speakers, by virtue, inter alia, of competitive pricing constraints. The latter characteristics give rise to low efficiency of MM speakers. FIG. 1 (taken from Ref. 3.), show eight frequency response curves of respective good quality eight commercially available MM speakers. As clearly shown, all speakers demonstrate a significant degradation in their efficiency below about 150 Hz.
Notwithstanding the specified limitations, the circulation of desk-top MM speakers have received a boost in recent years, due to the ever-increasing popularity of multimedia applications in the P.C. environment. The wide circulation of the multimedia speakers on the one hand and the relatively low performance thereof with respect to low frequency components of the sound signals, on the other hand, have encouraged developers to enhance the low frequency efficiency of the electro-acoustic transducers thereby to accomplish pseudo extension of sound signal.
There are available other prior art techniques which attempt to cope with the physical and psychoacoustic limitations as specified herein and a brief description of these solutions is given below.
Thus, one possible approach to cope with the physical limitation of the electro-acoustic transducers is simply to boost (by a fixed gain or by dynamically controlled gain) the low frequency component of the sound signal before it is driven to electro-acoustic transducer. This solution is rendered virtually useless in medium and low quality desk-top multimedia speakers, wherein the efficiency in low frequency range may drop below 1/100 (-40 dB) relative to the counterpart efficiency of the same electro-acoustic transducer at mid-high frequencies (see FIG. 1.). Thus, attempting to compensate for the difference in efficiency by amplifying the low frequency range of the signal necessitates a very high level of energy which may burn the electro-acoustic transducer or force the overall sound level to be impractically low. The drawback of the latter solution is further aggravated where lap-top multimedia computers are concerned, since the ability to compensate for low electro-acoustic transducers efficiency by boosting the energy necessarily increases power consumption. Increasing power consumption, is, of course, undesired in lap-tops, due to their limited battery capacity.
Various medium quality multimedia speaker systems employ a so-called sub-woofer. The latter is a separate device which is normally coupled to the conventional multimedia speaker and is employing a unique electro-acoustic transducer optimized for producing low frequencies. The sub-woofer is normally driven by its own power amplifier and hence it may greatly reduce the life span of the batteries that drive lap-top P.C. Moreover, sub-woofers are of relatively large size, and have normally high price tags. Thus, the price of sub-woofer systems may, in some cases, be as high as ten times the price of a conventional electro-acoustic transducer.
There follows a brief discussion in various prior art techniques, which unlike the aforementioned prior art techniques do not treat the low frequency signal directly, rather they offer alternative techniques to produce pseudo-LFPS.
Whilst the above citations accomplish certain level of Pseudo-LFPS they, generally, fail to come close to the low frequency psycho acoustical sensation since they do not treat adequately with all of the important attributes i.e. loudness, pitch and timbre.
In fact, none of these publications deals with matching timbre attribute and matching the loudness attributes.
Similar to the cited publications, the present invention also copes with the degradation of the LFPS by producing pseudo-LFPS, however, in departure from the specified citations, it deals properly with the specified attributes, by utilizing, inter alia, the known per se residue pitch effect and "equal loudness contours".
The equal loudness contours are depicted in FIG. 2 and are explained in detail in reference 1 (Chapter 2.3 pp. 45). As explained in the specified reference:
It is accordingly the object of the present invention to overcome or substantially eliminate the specified drawbacks associated with low efficiency electro-acoustic transducers insofar as conveying to a listener pseudo low frequency psychoacoustic sensation is concerned.